Oil and gas wells are typically drilled with a drill string having a drill bit on bottom that is rotated. One type of drill bit is a drag bit having blades with cutting disks that scrape against and cut the formation. Mud pumps on the drilling rig pump drilling fluid down the drill string and out nozzles on the bit face to sweep formation cuttings from the bit face. The drilling fluid entrains the cuttings and returns up an annulus surrounding the drill string. Particularly for horizontal wells, a mud motor may be to rotate the bit. Drilling fluid pressure powers the mud motor to rotate the bit independently of the drill string rotation. The mud motor requires a considerable pressure and flow rate of drilling fluid in order to be able to apply the desired torque to the drill bit.
If the cuttings are not readily removed, the rate of penetration of the drill bit declines. Bits may also plug and ball up while drilling sticky shale formations. If the mud motor is not able to rotate the drill bit at a desired rotational speed, the rate of penetration may decline. Many variations in the bit nozzle diameters, orientation and placement are used in order to more effectively remove cuttings.